Subzero: Fix incorrect address mode inference involving Phi temporaries.

Also, refactor the key part of the address mode inference into separate functions, since it's getting unwieldy.

The main thing is that we mark phi temporaries as multi-definition, and disallow address mode inference transformations that involve such temporaries, because this is incorrect particular when there are backward branches involved.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/557953007

tests_lit/llvm2ice_tests/phi.ll

diff --git a/tests_lit/llvm2ice_tests/phi.ll b/tests_lit/llvm2ice_tests/phi.ll
index ae96063..f9ec1ce 100644
--- a/tests_lit/llvm2ice_tests/phi.ll
+++ b/tests_lit/llvm2ice_tests/phi.ll
@@ -58,3 +58,53 @@
 
 ; ERRORS-NOT: ICE translation error
 ; DUMP-NOT: SZ
+
+; Test that address mode inference doesn't extend past
+; multi-definition, non-SSA Phi temporaries.
+define internal i32 @testPhi3(i32 %arg) {
+entry:
+  br label %body
+body:
+  %merge = phi i32 [ %arg, %entry ], [ %elt, %body ]
+  %interior = add i32 %merge, 1000
+  %__4 = inttoptr i32 %interior to i32*
+  %elt = load i32* %__4, align 1
+  %cmp = icmp eq i32 %elt, 0
+  br i1 %cmp, label %exit, label %body
+exit:
+  %__6 = inttoptr i32 %interior to i32*
+  store i32 %arg, i32* %__6, align 1
+  ret i32 %arg
+}
+; I can't figure out how to reliably test this for correctness, so I
+; will just include patterns for the entire current O2 sequence.  This
+; may need to be changed when meaningful optimizations are added.
+; The key is to avoid the "bad" pattern like this:
+;
+; testPhi3:
+; .LtestPhi3$entry:
+;         mov     eax, dword ptr [esp+4]
+;         mov     ecx, eax
+; .LtestPhi3$body:
+;         mov     ecx, dword ptr [ecx+1000]
+;         cmp     ecx, 0
+;         jne     .LtestPhi3$body
+; .LtestPhi3$exit:
+;         mov     dword ptr [ecx+1000], eax
+;         ret
+;
+; This is bad because the final store address is supposed to be the
+; same as the load address in the loop, but it has clearly been
+; over-optimized into a null pointer dereference.
+
+; CHECK-LABEL: testPhi3
+; CHECK: push [[EBX:.*]]
+; CHECK: mov {{.*}}, dword ptr [esp
+; CHECK: jmp
+; CHECK: mov
+; CHECK: mov {{.*}}[[ADDR:.*1000]]
+; CHECK: cmp {{.*}}, 0
+; CHECK: je
+; CHECK: jmp
+; CHECK: mov {{.*}}[[ADDR]]
+; CHECK: pop [[EBX]]